Extrude/scale on cylinder
Made without Booleans, this sort of topology stands up to pretty close scrutiny, (if you don't mind the holding loops around the tower).

The extra vertical loops in the cylinder that help define the curvature of the arch were created by selecting the relevant edge-rings, and using CtrlE > Subdivide Smooth. 

The arch can be formed  by selecting its central vertice(s) and in Proportional Edit, GZ, raising it/them in Z.

Then an inset and extrusions to create the holding perimeter and the thickness of the window, and an extra edge loop to hold the sharpness of its bottom corners.